In backscatter communications systems such as passive Radio Frequency Identification (RFID) tags, a device transmits a signal towards an antenna, and then measures the signal that is reflected back from the antenna. Each antenna must backscatter the frequency differently in order for the device to identify a particular antenna.
However, there are only a finite number of different backscattered signals that can be backscattered from the antennas and detected by the device. Antennas that have the same backscattering characteristics are difficult to distinguish from one another.
It would therefore be desirable to control the amount of backscatter emitted by an antenna, for example so that the backscatter of a particular antenna could be minimised to prevent it from being detected, or to prevent it from interfering with backscatter from a nearby antenna having similar backscattering characteristics.